A “Schmitt trigger” is an electronic circuit whose output stays constant until its input reaches one of two distinct threshold levels. Particularly, when the circuit's input signal overcomes a “high threshold level,” the output is switched from a first logic state to a second logic state. The output remains in this second logic state so long as the input voltage stays above a “low threshold level.” Then, when the input signal drops below the low threshold level, the output of the circuit returns to the first logic state.
In a non-inverting configuration, when an input voltage is higher than the high threshold level, the output of the circuit is at a logic high (e.g., “1” or “Vdd”). When the input voltage is below the low threshold level, the output of the circuit is at a logic low (e.g., “0” or ground). Moreover, when the input voltage is between the high and low threshold levels, the output of the circuit retains whichever of the two logic values it may already be in. In an inverting configuration, the complementary scenario is true—that is, when the input is higher than the high threshold level, the output is at a logic low, and, when the input is below the low threshold level, the output is at a logic high. The Schmitt trigger's “dual threshold action” is often referred to as “hysteresis.”
A conventional Schmitt trigger circuit suffers from process-voltage-temperature (PVT) variations when it operates near the rails—that is, close to the supply voltage (e.g., Vdd) or to the ground voltage (e.g., 0 V). Particularly, when a conventional Schmitt trigger's high and low threshold levels are set at values close to the rails (and therefore far from Vdd/2), the circuit tends to operate improperly. The inventors have determined that this problem is due, at least in part, to the fact that a conventional Schmitt trigger operates based upon voltage and/or current comparisons that are performed by a combination of P-type Metal-Oxide Semiconductor (PMOS) and an N-type MOS (NMOS) devices, each of which has different physical characteristics. Nonetheless, it would be desirable to provide a Schmitt trigger circuit having trigger thresholds with voltage values near the rails that is insensitive to PVT variations.